Among existing injection molding methods, a particularly well-known method is the resin transfer molding (RTM) method in which a mold comprises two half-shells that confine a mold cavity when they are placed one against the other. A fiber preform is inserted in the cavity between the two half-shells, and then resin is injected therein. The resin is polymerized while keeping the two half-shells closed. Depending on the desired production rate, polymerization may be performed at ambient temperature or by heating. Such a method can be used to make bodies of revolution, e.g. for fabricating fan casings of gas turbine engines for aviation.
The use of such a method is particularly beneficial since it enables parts to be made that present overall weight that is smaller than the weight of the same parts when they are made out of metal material, while still presenting mechanical strength that is at least equivalent if not greater.
On leaving the mold, i.e. on extraction of the fabricated part, deformation is commonly observed relative to the theoretical nominal shape. Thus, by way of example, for a body of revolution, such as a fan casing, a defect might be observed relative to the theoretical circular shape, which defect appears in the form of the part being ovalized on being extracted from the mold.
Such defects can be explained in particular by the fact that residual stresses act on the part during fabrication in the mold (e.g.: polymerization gradient, winding tension for a composite material part), which residual stresses are released when the part is extracted from the mold, thereby leading to deformation of the extracted part.
To counter that drawback, it is known to use at least one mold having a mold cavity of shape that does not correspond to the nominal shape of the part that is to be fabricated, but to a shape that takes account of the deformation, such that the part that is finally obtained on extraction from the mold has the nominal shape for the part. Beneficially, such a method makes it possible to counter the ovalization of a body of revolution on leaving the mold.
Nevertheless, when fabricating a fan casing, the ovalization observed therein does not arise solely while it is being extracted from the mold. Specifically, fabricating the casing subsequently involves various successive operations such as machining operations (e.g. trimming, drilling) and adhesive bonding (e.g. acoustic panels, fire protection panels). Machining operations lead to physical stresses being released that can encourage deformations of the casing. Adhesive-bonding operations are commonly performed in stoves. Such operations involve steps of raising the temperature of the casing and of putting it under pressure, followed by a step of cooling it. Together, those steps also lead to stresses appearing that act to ovalize the casing. Various ovalizations of the casing can thus appear throughout its fabrication process, and they tend to accumulate.
The existing state of the art thus appears to be insufficient for countering ovalization of the fan casing during its fabrication, given that the appearance of this ovalization does not occur solely while the casing is being extracted from its mold cavity, but also during steps of machining or of adhesive bonding.